Ceramic compositions of interest to the present invention include, for example, the one described in Japanese Unexamined Patent Publication No. 2002-173362 (Patent Document 1). The ceramic composition described in Patent Document 1 concretely contains 35.0 to 65.0% by weight (wt. %) in terms of SiO2 of a Si component, 20.0 to 50.0 wt. % in terms of BaCO3 of a Ba component, 5.0 to 35.0 wt. % in terms of MnCO2 of a Mn component, 3.0 to 10.0 wt. % in terms of Al2O3 of an Al component, 0.3 to 3.0 wt. % in terms of Cr2O3 of a Cr component, and 0.3 to 3.0 wt. % in terms of CaCO3 of a Ca component, and does not contain B2O3, namely a B component.
Since the B component is liable to scatter at the time of burning, it is necessary to conduct burning in a container-like sheath capable of enclosing the atmosphere for a ceramic composition containing the same, and hence, the size of a gap communicating between the inside and outside the sheath varies due to variations in accuracy of dimension and shape of the sheath, and as a result, variation can arise in composition of the ceramic composition after burning. However, such a problem will not occur in the ceramic composition described in Patent Document 1.
On the other hand, since a Mn component is contained in the ceramic composition described in Patent Document 1 instead of a B component that is liable to scatter, burning in a container-like sheath is not required, and thus burning can be conducted on a simple tray which is called a setter.
However, the flexural strength of the obtained sintered body with the ceramic composition described in Patent Document 1 is as low as about 150 to 200 MPa, which is not sufficient. Also, the Q value in the microwave band (for example, 3 GHz) is not sufficient, and a high Q×f value is not obtained.    Patent Document 1: Japanese Unexamined Patent Publication No. 2002-173362